UV curable organopolysiloxane compositions are well known in the art, for example, compositions predominantly comprising an organopolysiloxane having an aliphatic unsaturated radical (e.g., vinyl) attached to a silicon atom in its molecule and another organopolysiloxane having an organic radical containing a mercaptoalkyl (e.g., mercapto-propyl) radical attached to a silicon atom. These compositions are curable by exposure to UV radiation through addition reaction between the unsaturated radical and the mercapto radical. They are useful in a variety of applications including coating compositions for fiber optics.
The mercaptoalkyl radical-containing siloxane compositions, however, are less desirable from hygienic and working aspects partially because the inclusion of low-boiling components in the mercapto functional component causes the compositions to emit a highly stimulative sulfur odor inherent to the mercapto radical.
The organopolysiloxane compositions mentioned above have many working benefits including quick curing, one-part type without a need for catalyst mixing, and a sufficient working life. However, for example, a composition containing an organopolysiloxane in which the aliphatic unsaturated radical attached to a silicon atom is a vinyl or allyl radical and another organopolysiloxane in which the mercapto-containing organic radical attached to a silicon atom is a mercaptopropyl radical has a drawback that if the composition cured by UV exposure is allowed to stand under sunlight or UV radiation for a long time, the siloxane gradually degrades to such an extent that the cured product may eventually lose its original shape. This is a critical drawback for organopolysiloxane compositions of the mercaptoalkyl-vinyl (or allyl) addition reaction type, particularly when the above-mentioned organopolysiloxane composition once cured is used in an environment exposed to sunlight or UV radiation, for example, as a coating composition for fiber optics, an adhesive for glass lenses, and a coating composition for glass sleeves.